1. Field of the Invention
The present invention relates to a method of and an apparatus for concentrating, filtrating, separating and arranging particles and solutes in a fluid.
2. Description of the Related Art
It is necessary to exert some force on cells levitated in a solution without making direct contact with the cells with a view toward concentrating biomaterials such as the cells, etc. in non-contact form. An ultracentrifugation is known which uses gravity as the force referred to above. However, since it is necessary to rotate a tube with a sample held therein for a predetermined time at high speed upon its use with a view toward generating the gravity, a predetermined processing time interval is required regardless of any amount of a sample to be separated.
As a technique for directly capturing a very small quantity of sample, a technique using a light radiation force has been proposed by A. Ashkin et al. and has been patented as an optical trap device (see U.S. Pat. No. 4,893,886). However, light is not permeable into a solution in which particles exist therein in large quantity and whose particle density is high, and hence the present solution is not appropriate to concentrate a sample.
It has been reported by, for example, Junru Wu, "Acoustical tweezers", Journal of Acoustical Society of America, pp. 2140-2143, 89(5), May, 1991, saying an acoustic radiation force subjected to each particle, that when the acoustic radiation force is exerted on each particle, a polystyrene sphere having a diameter of 270 .mu.m could be successfully captured at a focal point of a focused ultrasound beam. The principle of capturing of the particles by the acoustic radiation force has been described in the reference of "Acoustic radiation pressure on a compressible sphere", pp. 167-173 and "Acoustic radiation pressure on bubbles and their logarithmic decrement", pp. 173-178, ACUSTICA vol. 5 (1955) by Yoshioka et al. According to the reference, the magnitude of an acoustic radiation force in an ideal fluid, that each particle undergoes in a standing wave and a traveling wave, has been estimated.
As has been also disclosed in Japanese Patent Application Laid-Open No. 7-47259, a method of introducing at least one ultrasound beam into a tube in which a fluid has flowed, thereby continuously concentrating the particles on a certain range, or a method of recovering the focused particles has been also invented. Further, a particle separating device described in Japanese Patent Application Laid-Open No. 6-241977, for utilizing an acoustic radiation force and an electrostatic force together in combination to thereby separate and recover particles different in particle diameter from each other or particles different in material from each other, and a particle separating device disclosed in U.S. Pat. No. 4,523,682, for using an acoustic radiation force and gravity in combination to thereby separate and recover particles different in particle diameter or density from each other or particles different in material from each other have been also invented.
The principle of applying ultrasound beams of different frequencies in a superimposed state to thereby effectively generate bubbles at a focal point in a fluid has been also described in the "Ultrasonic symposium proceeding", pp.1843-1846 (1994), IEEE reported by Umemura et al.